EXTERNAL ANATOMY OF INSECTS: 551 
which it bends into an ample sinus that receives the 
dorsolum *. . 
2. Dorsolum». Where there is no apparent collar, 
the dorsolum (dorslet) is the first piece of the mesothorax, 
and where there is one, the second; it bears the elytra 
or other primary organs of flight. It varies in the dif- 
ferent Orders, particularly with respect to its exposure. 
In Coleopterous insects it is most commonly, but not 
invariably °, covered intirely by the shield of the protho- 
vax, the scutellum alone being visible; as it is also 
in the Orthoptera (with the exception of Mantis and 
Phasma, in the first of which it is partially, and in the 
latter intirely exposed), and the Heteropterous, and 
most of the Homopterous section of the Hemiptera. 
The scutellum is likewise covered in Gerris, Hydrome- 
tra, and Velia, and the whole of the back of the alitrunk 
by a process of the prothorax in Acrydium, Centrotus, 
&c. But in the remaining Orders, and the tribe of 
Cicade in the Homopterous Hemiptera, the dorsolum 
is not hidden by the thoracic shield. It is usually less 
elevated than the scutellum ; in Necrophorus, and some 
other beetles, however, the latter is most depressed. 
With regard to its substance, it is generally not so hard 
and rigid as the scutellum, but in most Coleoptera harder 
than in the other Orders in which it is covered; in 
the Hemiptera, except in Cicada, it approaches to 
membrane. © As to shape and other circumstances, it 
@ Prare IX. Fic. 11. g’. b Prates VIII. TX. 7. 
* When the prothorax is separated from the elytra by a kind of 
isthmus, as in Scarites, Passalus, &c., the dorsolum is more or less 
uncovered. 
